The present invention relates in general to monitors for use in furnaces and other high temperature, dirty environments, and in particular to a new and useful monitor which is capable of determining the cleanliness of a furnace wall that receives deposits of highly reflective ash.
Western fuels, such as Powder River Basin (PRB) coals, are low in sulfur; however, they are high in calcium and silicon. This composition results in a thin, white tenacious ash on the furnace walls. This ash (called slag) is not easily removed using conventional air or steam cleaning devices.
A number of utilities are currently burning PRB coal and more are expected to switch to PRB coal with the passage of the Clean Air Act. The PRB coal deposited ash is highly reflective. The ash reduces the heat absorption of the furnace causing high temperatures in the convection passes of the boiler.
Waterblowing has been shown to be effective in removing the ash and restoring furnace heat transfer effectiveness. More utilities are expected to employ waterblowing when PRB coal is used.
The PRB ash deposit is thin and tight. This is in stark contrast to the heavy deposits visually apparent in furnaces burning other coals. Therefore, the decision to clean the furnace cannot be made based on a visual inspection and overcleaning (with water) is not desired because of thermal shock consideration.
Since there is a thermal shock consideration in using water, it is desirable to only clean the wall when cleaning is necessary. All plants currently clean on a time sequence. Most plants use a cleaning period of about four hours. Video recording taken at one plant reveals that for the hot wall (this was a tangentially fired boiler) four hours was an appropriate time period. However, on the cold wall a much longer period than four hours should be used. Based on the video recording even after ten hours the cold wall was still relatively clean.
A need thus exists for apparatus and techniques which are capable of accurately distinguishing between a clean condition for the furnace wall which does not require immediate waterblowing, and an unclean condition which does require waterblowing.